Conventional systems for detecting the presence of an oncoming train moving towards a level crossing for controlling the level crossing gate system have been relatively unsophisticated. Typically, a voltage between rails in an electrically isolated section is provided, and the conductive wheels of the train passing over the section allow for current to pass which is used to generate a signal for the level crossing gate system. While the reliability of such unsophisticated train presence detection systems is very high, the potential danger to human life by the failure of conventional systems makes it of paramount importance to provide detection apparatus which is as reliable as possible, if not 100% reliable.
In U.S. Pat. No. 4,179,744 to Lowe, a system for analyzing performance of electric traction motor powered railway locomotives is described in which the magnetic fields of electrical operating components of the electric traction motor powered vehicles are sensed. The results of the sensing are used for performance and maintenance evaluation purposes. While the speed of the train is obtained from the measurements, the system described measures the movement and operation of electrical operating components without providing useful information on the movement of vehicles containing no electrical operating components. While most trains in the United States have electric traction motors, it is possible for certain types of long freight trains to have locomotives in the middle or at the rear of the moving train. It is also possible for a train to have its traction motors turned off while still in motion. In the case that the locomotive at the front of the train is absent or turned off, detection of electrical operating components cannot be used as a reliable means for detecting the presence of a train moving towards a level crossing.
In U.S. Pat. No. 4,283,031 to Finch, a magnetic sensor for detecting the movement of a wheel of a rail car is described in which the speed and the direction of the rolling wheel can be determined. The wheel movement measurements from various sensors on each side of the level crossing are used to control the level crossing gate system. The wheel movement sensor disclosed in Finch is an active device mounted in close proximity to the moving wheel and is mounted above ground. By providing the sensor above ground and in a predetermined position adjacent the moving wheels of the train, the sensor is both exposed to the elements and exposed to risk of damage either by the train itself or by vandalism. The wheel sensor disclosed by Finch is not suitable for mounting at or below ground level.